1. Field of the Invention
The present invention relates to a magnetic recording medium, and particularly to an improvement of a magnetic recording medium of the type wherein a magnetic layer is coated on a transparent plastic base film.
2. Description of the Background
In recent years, home video recorders such as compact 1/2 inch home video tape recorders have become popular and have been spreading at a rapid rate. As the name "home video recorders" implies, such magnetic recording and reproducing devices are intended primarily for use at home and designed for easy operation. For instance, a home video recorder is designed so that in the operation of a magnetic tape mounted thereon for recording, reproducing, fast-forwarding or rewinding, the magnetic tape stops running when it reaches the end of the tape. Namely, such a home video recorder is provided with a means for detecting a light transmitted through a transparent leader tape portion provided at the end of the magnetic tape, and thus is designed to stop the tape from running upon detection of the transmitted light. The means for detecting the transmitted light is likely to detect also lights from other light sources than that of the home video recorder. Therefore, the light detecting means is so designed that it does not operate unless it detects a substantial quantity of light. For instance, if the magnetic tape is adapted to stop running when the photo transistor current due to the light transmitted through the magnetic tape is as low as about 1 .mu.A, there will be frequent accidental stopping of the running tape upon the detection of lights from light sources other than the home video recorder even when the quantity of lights transmitted through the magnetic tape is relatively small. Accordingly, the magnetic tape is so adapted that it does not stop running until the photo transistor current due to the light transmitted through the magnetic tape reaches a level of about 3 .mu.A. Accordingly, the magnetic tape must be designed so that the light will not be transmitted through the magnetic recording and reproducing portion. For this purpose, there may be (1) a method wherein the magnetic layer serving to prevent the transmittance of light to a certain degree, is made thick, (2) a method wherein the base film for the magnetic tape is made not to transmit a light, or (3) a method wherein a substance inhibiting the transmission of light is incorporated in a substantial amount in the magnetic layer so that it is effective even when the magnetic layer is relatively thin. Among these methods, method (1) has a drawback that when the magnetic layer is made thick, the loss in the recording efficiency due to the diamagnetic field becomes great. Further, if the magnetic tape is made thick, it is required to take a large space for its accommodation. Accordingly, in the case of a home video recorder having a relatively compact structure, the thickness of the magnetic tape can not be made so thick. Therefore, it the magnetic layer is made thick and the base film is made thin, the toughness of the magnetic tape decreases, whereby there will be a drawback that the magnetic tape is susceptible to rupture or deformation. In the case of method (2), carbon powder or the like maay be incorporated in the base film. However, it is rather difficult to prepare a base film wherein carbon powder is uniformly distributed. It has an additional drawback that the production cost will be high. With respect to method (3), it has been proposed to incorporate a substantial amount of carbon powder in the magnetic layer. However, in order to adequately inhibit the transmittance of light by means of e.g. carbon powder, it is required to incorporate at least about 10% by weight of carbon powder relative to the magnetic powder such as .gamma.-Fe.sub.2 O.sub.3, and otherwise no adequate effectiveness is obtainable. However, if a substantial amount of carbon powder is employed, the S/N ratio of the magnetic tape deteriorates substantially. If a magnetic tape is prepared by using hardly light-transmitting Fe.sub.3 O.sub.4 instead of .gamma.-Fe.sub.2 O.sub.3 as the magnetic powder, the Fe.sub.3 O.sub.4 is unstable to a temperature and susceptible to oxidation. Therefore, Fe.sub.3 O.sub.4 was not desirable as the magnetic powder for a magnetic recording medium. Further, for the video tapes, high density recording and long hour recording are required, and there is a tendency such that the magnetic powder is required to be finer and the tape is required to be thinner, i.e. the tendency is adverse to the inhibition of the transmittance of light. Namely, Co-containing .gamma.-Fe.sub.2 O.sub.3 is commonly used for the video tapes, and if the particle size is made finer, the transmittance of light increases, and the transmittance further increases by making the video tape thinner.